1. Field of the Invention
This invention relates to a shield structure for shielding various kinds of electronic circuits and more particularly to an improved shield structure for providing effective electromagnetic shielding for a prescribed region of a circuit board having circuit elements mounted thereon.
2. Prior Art Statement
For the purpose of realizing an electronic circuit with specific desired functions, it has become an extremely commonplace technique to mount various electronic components on an appropriate circuit board, typically a printed circuit board, and to electrically interconnect the components by a network of conductive lines which may, for example, be printed wires.
It has also become an ordinary practice to enclose both the electronic circuit constituted in this way and the circuit board on which it is mounted within an appropriate housing of, or lined with, a conductive material, in this way making the circuit resistant to external electromagnetic interference. There are, however, cases in which it becomes desirable to provide further electromagnetic shielding for specific, limited regions on the surface of the circuit board.
While there are any number of cases in which this type of electromagnetic shielding is required, there will be taken up as an easy-to-understand example the radar detector produced by the present inventor.
Radar detectors are in particularly wide use in the United States. When the radar detectors detects an X-band or K-band microwave signal (what is generally referred to as a "radar wave"), such as transmitted by the police for enforcement of vehicle speed regulations, it produces a warning sound or causes a lamp to go on or off. Typically such radar detectors are of the double conversion type and comprise a circuit block of the general type shown for a radar detector 9 in FIG. 1.
The radar wave is received by a horn antenna 1 and the output from the antenna 1 is applied to a first mixer 2 which also receives a first local oscillator frequency produced by a first local oscillator 3. The output of the first mixer 2 is sent through a first intermediate frequency (IF) amplifier 4 to a second mixer 5 which receives a second local oscillator frequency produced by a second local oscillator 6, and the output from the second mixer 5 is forwarded through a second IF amplifier 7. After the signal has been converted to a fixed second intermediate frequency in this way, it is applied to a discriminator or detector 8.
As regards the specific frequencies employed in the circuit of this structure, the center frequency of the X-band is 10.525 GHz and that of the K-band is 24.150 GHz, and in either case a band width of .+-.100 MHz relative to the center frequency is permissible.
For enabling the radar detector 9 shown in FIG. 1 to detect both X-band and K-band signals, it is known to employ the harmonic mixing system in which a basic frequency of, for example, 11.558 GHz is used as the first local oscillator frequency.
More specifically, in the case of the X band the beat frequency with the basic frequency is extracted from the first mixer 2, and in the case of the K band the beat frequency with the second harmonic of the basic frequency (11.558.times.2=23.116 GHz) is extracted from the first mixer 2. Thus, in either case, an intermediate frequency of 1,033.+-.100 MHz is output by the first IF amplifier 4.
This intermediate frequency is further beat down in the solid state second mixer 5 so as to convert it to a signal with a frequency of several tens of megahertz, which is applied to the signal detector 8. When the signal detector 8 detects a signal with a frequency on the aforesaid order, it drives an appropriate sound generating means or visual display means (neither shown), in this way producing a warning.
The biggest problem encountered in a circuit system of this type is that of the interference of external noise with the circuit signals. The effect of such noise is particularly large on the portion of the circuitry that handles the first intermedaite frequency, because the first IF amplifier is generally constituted as a low-noise, high-gain amplifier. Such noise may well cause erroneous detection by the final signal detector 8.
Another problem is that the 1,033.+-.100 MHz band used as the IF frequency in the radar detector 9 is also used as a communications carrier frequency by other types of communication circuit systems and the like. As a result, radar detectors of this type are frequently subject to interference from such communications equipment.
In view of this situation, those portions of the circuit that are easily or strongly affected by such interference (specifically, the first mixer 2, the first IF amplifier 4, the second mixer 5 and the second local oscillator 6) have been designated as a "shield-requiring region" 10, and this region 10 has been provided with a special shield in addition to the overall shield provided for the radar detector as a whole.
When, as in the foregoing example, it is found necessary to designate a certain portion of the circuitry on a circuit board as a shield-requiring region, the most common method used for providing the special shield has been as will now be explained with reference to FIG. 2(A).
In FIG. 2(A), reference numeral 11 designates a circuit board, typically a printed circuit board, made of an insulating material and an electronic circuit (not shown) is fabricated on the surface of this board. The area corresponding to that part of the circuit designated as a shield requiring region 10 is indicated by hatching. In the conventional shielding method, the perimeter of the shield requiring region 10 is enclosed by a conductive pattern 12 formed on the surface of the circuit board 11. As schematically illustrated in the figure, the conductive pattern 12 is connected with ground point E via an appropriate current path.
A metallic box member 13 open at one side is constructed to have a height slightly greater than the maximum height of the electronic components mounted on the circuit board 11 and to have the shape of the edge defining the open side conform to the shape of the conductive pattern 12. The metallic box member 13 is set on the circuit board 11 with said edge resting on the conductive pattern 12.
Then, as shown in FIG. 2(B), the region bridging the line of contact between the lower edge of metallic box member 13 and the conductive pattern 12 is subjected to soldering 14 so as to electrically and mechanically fix the metallic box member 13 with respect to the conductive pattern 12 and thus obtain a fixed shield structure.
Alternatively, as shown in FIG. 3(A), the metallic box member 13 is provided along its lower edge with a number of tabs 15, and the metallic box member 13 is fixed on the circuit board 11 by screw-fixing means 18 constituted either of bolts 16 passed through holes in the tabs and the circuit board 11 and having nuts 17 screwed thereon or of the bolts 16 an tapped holes provided directly in the circuit board 11. In either case, the metallic box member 13 and the circuit board 11 are forced onto each other to obtain a mechanical fixing force therebetween, and, at the same time, the edge of the metallic box member 13 defining the open side thereof is pressed onto the conductive pattern 12 to esablish an electrical shield structure.
However, this conventional shield method has a major intrinsic defect.
In the method illustrated in FIGS. 2(A) and (B), a considerable amount of time and labor is required for providing the soldering 14 around the entire periphery of the lower edge of the metallic box member 13. The need to provide this soldering thus greatly interferes with production efficiency.
Moreover, the heat used for fusing the solder is apt to collect within the metallic box member 13 and may thermally damage the components of the circuit mounted on the shielded region 10.
In the screw fastening method illustrated in FIGS. 3(A) and (B), on the other hand, while the fastening operation can be carried out more efficiently than in the case of soldering, it is found that the method frequently results in incomplete shielding. More specifically, the metallic box member 13 is generally fabricated by punching or bending a thin steel or copper sheet so that the edge defining the open side thereof is not, as shown schematically in FIG. 3(C), absolutely flat when viewed microscopically. Moreover, while not illustrated in the figure, the circuit board 11 also has a slight warp and does not exhibit such a high degree of flatness. As a result, a gap 19 is apt to occur between the bottom edge of the metallic box member 13 and the conductive pattern 12.
Also, when the screw fastening is carried out using tabs provided at several places along the bottom of metallic box member 13, since the amount of fastening force is different between those portions directly fastened by the screws and the portions intervening between the screw-fastened portions, warping of the edge defining the opening of the metallic box member 13 is likely, whereby similar gaps 19 are likely to occur.
The presence of such gaps naturally has a degrading effect on shielding performance and the problem of incomplete shielding has been found to actually occur with the conventional structure.